There is a class of semiconductor products that are predominantly planar and specular (flat and shiny), and it is frequently necessary to image these devices in such way that even minor deviations from planar and specular are imaged with adequate contrast. One such class of products includes semiconductor wafers that may be provided with indicia indicating, among other things, wafer number and manufacturer. These indicia are defects in the surface of the wafer and are typically a matrix of laser etched pits. They are commonly known in the art as “soft marks” or “fiducial markers.” It is necessary to image these marks to read the codes at various steps along the manufacturing process.
Another class of products that are routinely processed using laser-power material removal schemes includes dielectric-coated conducting metal substrates used as chip carriers for semiconductor devices or as printed wiring boards for electronic devices. These, too, are predominantly planar and specular (flat and shiny), and it is frequently necessary to image these devices in such way that surface and/or subsurface features can be imaged with adequate contrast. One such class of products includes semiconductor chip carriers that may be provided with indicia indicating, among other things, process datums. These indicia are generally conducting metal (commonly copper or a copper alloy) on or beneath of the surface of the workpiece and are typically round, square or another geometric shape created by chemical etching the metal conducting layer during earlier processing step(s). But, these indicia could include mechanically drilled or punched holes through the entirety of the workpiece. They are commonly known in the art as “tooling holes,” “fiducial marks,” or simply “alignment targets.” It is necessary to image these marks to align and scale the workpiece to the machine at various steps along the manufacturing process.
After the devices have been processed (generally cut by saw into individual rectangular devices), it may be necessary to inspect the edges for small chips and cracks that may propagate over time and cause premature device failure or to inspect the laser-processed features for cosmetic or functional reasons. These inspection processes are automated and use electronic imaging cameras in combination with digital electronic computers that are programmed to perform the necessary inspections, measurements and identifications.
To properly image these objects, including highlighting these very minor features, a number of lighting systems and cameras have been used. For example, commonly assigned U.S. Pat. No. 5,737,122 entitled Illumination System for OCR of Indicia on a Substrate, which is incorporated herein in its entirety by reference. The '122 patent describes a system wherein the axis of the camera and the axis of the lighting module are at complimentary acute angles symmetric about the normal to a specular object. The narrow-angle dark field lights are positioned close to the optical axis and are prevented from being directly imaged by the camera by baffles placed in the imaging path to prevent this. The position of the baffles restricts the field of view of the imager, but this is considered an acceptable compromise.
In another example, commonly assigned U.S. Pat. No. 6,870,949 discloses a coaxial narrow angle dark field imaging system that utilizes a telecentric lens to illuminate objects with symmetric coaxial narrow angle dark field illumination. The illumination technique is particularly suited to highlight minor features or defects on planar specular objects. In particular, the coaxial light source directs light rays towards a telecentric lens which redirects the light rays towards the substantially planar specular object. The light rays are reflected back through the telecentric lens towards a camera. To the extent that the light rays are reflected from a planar specular portion of the object the light rays are blocked by a telecentric stop. Light rays reflected from a defect or feature in the planar specular object will pass through an aperture in the stop to a camera. U.S. Pat. No. 6,870,949 is also incorporated herein in its entirety by reference.
With these and other systems, images are obtained using monochromatic cameras are typically shown in grey-scale. Lights used are white or monochromatic lights, such as white or single wavelength LEDs. Using these images, automatic identification of fiducial markers on a work piece for inspection and part alignment purposes is a well-established practice in the machining industry. Various algorithms have already been developed and are readily in use to process the images for this identification. Regardless of how sophisticated an algorithm is, however, a “good” image with a high level of contrast between the target and the surrounding background is still critical to its success.